PURPOSE: To enable to correspond to the increase in the running load by providing an alternative switching clutch between the first and second transmission states, an automatic switching means for operating the switching clutch and an artificial switching preventing means for preventing the automatic switching operation.

CONSTITUTION: When a speed change operating means 22 is operated to the low speed side, the second transmission state is automatically obtained and the direct motive power is supplied to the traveling group from a hydraulic pump P so that the output corresponding to the load can be obtained. Moreover, as a means B for preventing an automatic switching means A from operating at the time of normal load running is provided, the speed change operating means 22 is not switched to the second transmission state even in operating the means 22 to the low speed side. The fine speed change operation can be thus obtained by a static-pressure type continuously variable transmission as a main transmission gear and a sub-transmission gear while the first transmission state is maintained.